The present invention relates to a process and apparatus for the continuous casting of a liquid polymerizable composition. In particular the present invention relates to a process in which the liquid polymerizable composition is continuously fed into a spacing defined between a pair of moving endless belts and formed into plates.
Cell casting is in general known as a process for the preparation of plates of polymers from polymerizable monomers by the application of heat or light. The cell casting has been employed, for example, for the preparation from methyl methacrylate of plates of methacrylic resins having desirable properties including transparency, surface gloss and weatherability. The cell casting is a method in which a liquid monomer such as, for example, methyl methacrylate or a syrup containing in part its polymerized compounds is cast in a cell into which two sheets of reinforced glasses are assembled with their outer peripheries sealed with flexible gaskets. The cell casting process involves polymerization of said liquid monomers fed into such a cell that is immersed in a hot water bath or in a hot air oven and the stripping of the resulting plates from the cell by separating the assembled cell. Accordingly, the applicability of conventional cell castings may be largely restricted by the size of the glasses used. This method involves a series of staged operations such as the cell assembly, the feeding of liquid monomers and the separation of the assembly, so that it requires a great deal of manual work. Consequently, this method raises the cost of product, and there has been a growing demand for continuous casting process.
Some continuous casting processes are known, for example, U.S. Pat. Nos. 2,500,728, 3,371,383, and 3,376,371. Japanese Patent Publication No. 41,602/1971, and Canadian Patent No. 895,406. Each discloses a process for the preparation of a plate of a polymeric material which comprises filling with a polymerizable material a spacing defined between a pair of endless belts, the first belt being disposed above the second belt, and completing polymerization during the movement of the belts. These processes, however, result in a non-uniformity of the thickness of the resulting plates. Particularly, the main difficulty involved in conventional continuous casting processes is the loss of control over the thickness of the final plate product in a widthwise direction. For example, Canadian Patent No. 895,406 discloses a process in which one of the two belts is so curved that the pressure applied by the other belt is made uniform, thus preventing the warping, waving or rippling of the belts. However, the process disclosed therein is not satisfactory in that control of the precise thickness of the final product is not achieved and the surface quality of the final product, particularly in the widthwise direction, is impaired.
It is therefore the primary object of the present invention to provide a process for the continuous casting of a liquid polymerizable composition which can overcome or reduce to a considerable extent the disadvantages of prior art continuous casting processes.
Another object of the present invention is to provide a process for continuously casting a liquid polymerizable composition which comprises feeding the liquid polymerizable composition into a spacing defined between a pair of moving endless belt conveyors, polymerizing it partially to a viscosity in a specified range, passing the partially polymerized composition through a path which is arranged so as to curve at least once, until a degree of polymerization of the resulting partially polymerized composition reaches a predetermined value, and then passing it through a path which is arranged in a straight direction for completion of the polymerization of the partially polymerized composition.
Another aspect of the present invention is an apparatus for continuously casting a liquid polymerizable composition by the process according to the present invention.